Anne-Marie Slaughter argues in Unfinished Business, her hot new book due out Tuesday (read an excerpt here), that corporate America and the U.S. government need to enact new policies and practices to help women advance. Kudos to Slaughter, who laid out the problem brilliantly in her 2012 Atlantic article, “Why Women Still Can’t Have It All,” and now continues to serve a lot of women well. But her advice misses a key part of the solution for getting more women into leadership positions.

Here’s why: Even if companies provided utopian work environments for women, dropouts and “mommy trackers” would still quit the upper ranks. I have a good sense of this because I’ve overseen Fortune Most Powerful Women since its 1998 launch and I’ve interviewed hundreds of women leaders since. To most highly educated, high-potential women, success means much more than holding a top job. Women tends to view power horizontally—it’s about impacting many things broadly—vs. climbing the ladder, which is generally more of a turn-on to men. “Power is the ability to impact with purpose,” Oprah Winfrey told me years ago when I asked her for her definition. It’s a definition that many woman covet.

Moreover, female concepts of power and success develops early. Women—even as they comprise nearly 60% of students entering top U.S. universities and tend to outperform men in the classroom—are up to 50% less likely than their male peers to enter competitive fields after graduating, notes Sally Blount, the dean of the Kellogg School of Management at Northwestern University. By competitive fields, she means areas such as investment banking and management consulting, which tend to be the best training grounds for big jobs later. One year out of college, women, on average, earn only 82% of what men earn.

So, many high-potential women opt out—or “opt lower” may be the more accurate term—practically before they begin their career. Two other pivot points follow. Most women in their 30’s “focus on how much easier tomorrow would be if they were not enduring the working-mother juggle,” says Blount, who is a psychologist by training. “In decision-making, we say they’re overweighting short-term benefits at the expense of the long-term—that is, their years after age 50, when they’ll want to be doing something interesting and challenging out in the world.” The third pivot occurs if a woman’s career trajectory has carried her to the cusp of senior leadership. “You have probably met as I have,” Blount tells me, “so many talented women who do make it through in their career quite successfully into their early 50s, then retire to sit on boards before they hit the C-suite.”

Indeed, I have. In my 31 years at Fortune, I’ve talked with scores of female executives who lean in and run hard to a certain level (of near-exhaustion) but then have zero interest in heading the companies that employ them. This is one of the reasons we’ve gone from two female CEOs of Fortune 500 companies in 1998 to only 23 today; women still lead fewer than 5% of America’s largest companies. Many of the women who could fill those CEO jobs leave the corporate world to do philanthropic or other work that is, in their minds, more meaningful than managing thousands of employees and catering to Wall Street.

While new policies and practices and a “universal infrastructure of care” comprise Slaughter’s wish list to fix gender disparity, I wish women would get more comfortable with ambition and power. According to a recent survey by TIME and Real Simple, Fortune’s sister titles within Time Inc. TIME , 51% of men and 38% of women said they would describe themselves as “very or extremely ambitious.”

Now, if anyone belonged in that 38% Club, you would think it would be the super-accomplished types who appeared on stage at a recent TIME/Real Simple event in Manhattan. Yet when NBC Today co-host Samantha Guthrie was asked “Are you ambitious?” she replied, “I hate the word.” Guthrie, who is a lawyer and a new mother in addition to her TV duties, went on to say, “The one thing I’m ambitious about is not failing.”

Another speaker at the event, U.S. Ambassador to the United Nations Samantha Power, said that she’s “sensitive about appearing too ambitious” because “the word ambitious tends to connote a sort of titular advance.” (The outlier was U.S. Sen. Claire McCaskill (D-MO), who proclaimed, “I am ambitious. Really ambitious.” But she cautioned that aspiring women tend to be judged more harshly than men are, and she suggested that Carly Fiorina had best smile more often if she wants a shot at the White House.)

Sheryl Sandberg would not approve (nor do I), but we still live in a world where successful women tone it down, while successful men can be themselves. Mary Wittenberg, the former CEO of the New York Road Runners who is now global CEO of Richard Branson’s Virgin Sport, told me that upon leaving the TIME/Real Simple event, she bumped into two “super-talented guys in their late 20s” whom she had worked with at NYRR. “I asked them, ‘Do you consider yourself ambitious?’ I got an immediate ‘yes’ from one, and with only the slightest pause from the quieter guy, a strong ‘yeah.’”

That’s a focus group of two, but it’s telling.

Says my colleague, Nina Easton, who is a Fortune senior editor and columnist, chair of Fortune Most Powerful Women International, TV political analyst, and a mother of three (yes, she’s ambitious): “A missing piece of this conversation is how many highly educated, top-talent women drop out, curtail their work, or (like me) choose a ‘mommy-tracker’ route in their careers—not because of discrimination or hostile work environments but because of the time they want to devote to their kids.” Easton and other women like her are making a “realistic personal choice,” she says, adding, “By definition, this limits the pool of female talent at the very top.”

And while some women on the fast track have the magical marital setup (Slaughter asked some 400 women leaders at the 2013 Fortune Most Powerful Women Summit how many of them had husbands holding down the fort at home, and 60% raised their hands), most women lack such a luxury. And no amount of paid parental leave or flextime enables true work-life balance. “What good is six weeks or six months of paid family leave when a manager goes back to a 24/7 work environment when it’s over?” says a close friend of mine, who abandoned a promising career years ago to raise her three kids. “What good is a day a week at home if your other four days are 16 or 18 hours?

My message to Slaughter: While changes in policies can benefit the work lives of working poor and middle-class women, there is only so much that companies and the government can do for executive-level women. So, what can we do to help propel more women into top leadership positions? We can start by better defining and modeling ambition for women and girls. Says Wittenberg, “We need to ensure that the connotation captures the positives of the concept—making a difference in the world and being one’s best self.”

That’s a fine concept for anyone to buy into and a foundation to build on.